Infrared optical filters have been used for a variety of instruments and applications to filter or separate light energy. Typical uses include UV curing units, scanning instruments and other sensing applications as shown in the sales literature, SWP & LWP Filter Coatings for Glass Substrates, which is provided by Evaporated Coatings, Inc., for example.
Image sensor arrays typically comprise a linear array of photosensors which raster scan an image bearing document and convert the microscopic image areas viewed by each photosensor to image signal charges. Following an integration period, the image signal charges are amplified and transferred as an analog video signal to a common output line or bus through successively actuated multiplexing transistors. One example of such an array is a charged-coupled device (CCD).
For high-performance image sensor arrays, a preferred design includes an array of photosensors of a width comparable to the width of a page being scanned, to permit one-to-one imaging generally without the use of reductive optics. In order to provide such a "full-width" array, however, relatively large silicon structures must be used to define the large number of photosensors. A preferred technique to create such a large array is to align several butted silicon chips, each chip defining a small linear array thereon.
The silicon chips which are butted to form a single full-width array are typically created by first creating the circuitry for a plurality of individual chips on a single silicon wafer. The silicon wafer is then cut or "diced," around the circuit areas to yield discrete chips. Typically, the technique for dicing the chips includes a combination of chemical etching and mechanical sawing. On each chip, the photosensors are spaced with high resolution from one end of a chip to the other; the length of each diced chip from one end of the array thereon to the other requires precision dicing. It would be desirable to dice each individual chip with a precise dimension along the linear array of photosensors, so that, when a series of chips are butted end-to-end to form a single page-width linear array, there is a minimum disruption of spacing from an end photosensor on one chip to a neighboring photosensor at the end of a neighboring chip. Ideally, the spacing, or pitch, across an entire full-width linear array should be consistent regardless of the configuration of silicon chips forming the array.
Preferably, the full-width array extends the entire length of a document, such as eleven inches. Usually, the full-width array is used to scan line by line across the width of a document with the document being moved or stepped lengthwise in synchronism therewith. A typical architecture for such a sensor array is given, for example, in U.S. Pat. No. 5,473,513. When the original document moves past the full-width array, each of the photosensors converts reflected light from the original image into electrical signals. The motion of the original image perpendicular to the linear array causes a sequence of signals to be output from each photosensor, which can be converted into digital data.
With the gradual introduction of color-capable products into the office equipment market, it has become desirable to provide scanning systems which are capable of converting light from full-color images into separate trains of image signals, each train representing one primary color. In order to obtain the separate signals relating to color separations in a full-color image, one technique is to provide on each semiconductor chip multiple parallel linear arrays of photosensors, each of the parallel arrays being sensitive to one primary color. Typically, this arrangement can be achieved by providing multiple linear arrays of photosensors which are physically identical except for a selectively translucent primary-color overlay over the photosensitive areas, or "photosites," for that linear array. In other words, the linear array which is supposed to be sensitive to red light only will have a translucent red layer placed on the photosites thereof, and such would be the case for a blue-sensitive array, a green-sensitive array, or any other color-sensitive array. These translucent layers are also referred to as absorption filter layers, because they selectively adsorb or block light having certain frequencies or wavelengths from reaching the photosensitive areas. Although it is preferable to use three linear arrays, any number of linear arrays can be used. As the chips are exposed to an original full-color image, only those portions of the image, which correspond to particular primary colors, will reach those photosensors assigned to the primary color.
The most common substances for providing these selectively translucent filter layers over the photosites are polyimide or acrylic. For example, polyimide is typically applied in liquid form to a batch of photosensor chips while the chips are still in undiced, wafer form. After the polyimide liquid is applied to the wafer, the wafer is centrifuged to provide an even layer of a particular polyimide. In order to obtain the polyimide having the desired primary-color-filtering properties, it is well known to dope the polyimide with either a pigment or dye of the desired color, and these dopants are readily commercially available. When it is desired to place different kinds of color filters on a single chip, a typical technique is to first apply an even layer of polyimide over the entire main surface of the chip (while the chip is still part of the wafer) and then remove the unnecessary parts of the filter by photo-etching or another well known technique. Typically, the entire filter layer placed over the chip is removed except for those areas over the desired set of photosites. Acrylic is applied to the wafer in a similar manner. After the chips are mounted to a substrate as taught in U.S. Pat. No. 5,473,513, a glass cover is placed over the chips and mounted on the substrate to provide physical protection of the chips. In the prior art, the glass cover is clear and transmits all light including infrared light. However, infrared light received by the photosites can be mistakenly interpreted as visible light by the photosites, which decreases image quality.